Large difference in chromosomes (number and characteristics) within Choloepus may mean there are even more than 2 species
of two-toed sloths

Two-toed and three-toed sloths were formerly placed in the same family but the two genera have profound behavioral and anatomical differences and are believed to come from two different fossil lineages. They are now placed in separate families.

2-toed is larger, faster, and nocturnal. Diet is more varied - eats leaves and fruit. 6 or 7 neck vertebrae and vestigial tail

3-toed is smaller, slower and both diurnal and nocturnal. Highly specialized browsers - eat only leaves. 8 or 9 neck vertebrae. Stout tail is 68 mm (2.7 in ) long

All sloths have 3 claws on their hindlimbs

Sloths are more closely related to anteaters than armadillos. Members of this group of animals have a reduced number of teeth.

Anteaters are toothless

Sloths have only 10 upper teeth and 8 lower teeth

Anteaters and sloths were once grouped together as "Edentata" meaning 'without teeth'.

Found in South, Central, and North America, Caribbean and Antarctica. Size varied from a medium dog to elephant. (Nowak 2008)

Phylogenetic relationships are not resolved. Gaudin and McDonald postulate four families: Megalonychidae, Megatheriidae, Nothrotheriidae and Mylodontidae. Bradypus is placed as a sister taxon to all other sloths. Choloepus is incorporated into the Megalonychidae.

At least three genera of giant ground sloths lived in Southern California during the Pleistocene. A Shasta's ground sloth skeleton was recently excavated in Carlsbad (thought to be about 2.1 m (7 ft) long and weighing about 136 - 181 kg (300 - 400 lbs) — bear sized.

First sloths arrived in North America about 7 million years ago (presumably by swimming between islands from South America)

By 13,000 years ago, all ground sloths were extinct except for a few populations on Caribbean islands.

Northernmost population ranges from Nicaragua south into western Venezuela

Southern population is found from north-central Peru, through extreme southwestern Brazil (southwestern Amazon and probably Acre) to central Bolivia.

There is a doubtful record for this species from the Aripuana, Mato Grosso State, Brazil (F. Avilla-Pires, pers. commun). The range of the species within Brazil is unclear and further surveys are needed.
Native to: Bolivia, Brazil, Colombia, Costa Rica, Ecuador, Honduras, Nicaragua, Panama, Peru, Venezuela

Habitat:

Limited to humid, warm, well-established tropical and cloud forests

Trees with interlacing crowns allow for lateral movement without descending to ground (descend approximately weekly to defecate)

Prefer crowns of trees with lianas to provide cover from predators and shelter from sun during the day.

They cannot shiver to keep warm as other mammals do because of the unusually low metabolic rates and reduced musculature. They have the lowest muscle mass relative to overall body weight of any mammal.

Two-toed sloth is exclusively nocturnal (three-toed sloth is active day and night)

Activity begins about one hour following sunset

Active about 11 hours throughout night

Activity ceases about 2 hours before dawn

Average 7.6 hours of activity/day

55% of time is spent in bouts of continuous activity lasting 2 or more hours

Sleep 15 to 18 hours per day

Approximately 6 hours each day are spent foraging

Most two-toed sloths change to a different tree each night

Does everything hanging upside down — eating, sleeping, mating and even giving birth!

Basal metabolic rates lie between 40 and 60% of that expected from their mass

Social Groups

Typically solitary; females occasionally feed in the same tree

Young will stay with mother for nine to twelve months, depending on the species (Taube 2001)

Aggression

Charges suspected aggressor, pulls objects to mouth with forearm and bites — sharp teeth are like canines of carnivores

Three-toed sloth is more docile

Their front teeth are not sharp so they don't bite defensively

They are often taken for pets because of mild disposition

CommunicationTactile

Mutual grooming observed only during infant/mother relationship —
mother vigorously licks head, face and ano-genital region for first few weeks of infant's life. Stimulates young to pass urine and feces. (Meritt 1985)

Self grooming/scratching with fore claws

Vocalization

Generally silent; hiss in defense; low bleats in distress

Sloth infants separated from their mothers will let out a loud bleat lasting 30-90 seconds

Mating occurs throughout the year, though some observers detect a marked mating season in March and April

Females in estrus appear to initiate mating (Meritt 2985)

Reproductive Rate

Inter-birth intervals:

C. hoffmanni about 15 months: young independent at 10 months

C. didactylus about 16 months; young independent at 12 months

Females do not show a decline in fecundity as they age (Nowak 1999)

Gestation

Similar in both Choleopus species: 10 months (Taube 1985)

Bradypus gestation about 6 months

Life StagesBirth

Litter size: one

Weight: 340 - 400 grams (about 12 ounces)

Length: 25.4 cm (10 inches)

Mother gives birth on ground or in upside down, hanging position; infant grabs onto her fur and makes its way to her chest to nurse

Milk is higher in fat (6.9 %) and protein (61%)
than cow's milk

Infant (< 1 year old)

Infant is born alert and strong, eyes open teeth present, claws fully formed. Gripping reflex enables it to climb to mother's abdomen. Nuzzling and suckling by newborn stimulates release of milk — usually within 48 hours

Newborn nurses for 6015 days for male infant, 27 days for female

Play behavior observed at 15-19 days

First hangs upside down at 20-25 days, and regularly feeds away from mother at 5 months

Young of all species cease nursing at about a month old, but may take leaves even earlier

Young are carried on mother's abdomen for six to nine months and feed on leaves they can reach from the position

By 6 months, elimination occurs in adult manner

May keep a close association for up to two years

Adult

Two-toed sloths reach sexual maturity at approximately three years of age (females) and four to five years old (males)

External genitalia small and inconspicuous (sexing is difficult)

Longevity (Nowak 1999)

Two-toed sloths live 10 to 15 years in the wild and over 30 years in zoos.

Note: The National Zoo has a 40 year-old Linné's Two-toed Sloth (Stewart 2004)

2006 assessment by Meritt, M. & members of the Endentate Specialist Group was LC (Least Concern):

Choloepus didactylus: LC status because of wide distribution, and occurrence in a number of protected areas. Population is not declining fast enough to quality for listing in a more threatened category. Far more widespread than C. hoffmanni

Choloepus hoffmanni - LC because of wide distribution, presumed large population, occurrence in protected areas, and tolerance to habitat modification. because of ongoing deforestation, the northern population (Nominate subspecies) of this species could potentially be assessed as Near Threatened

Of the four Bradypus species, two are listed at LC (B. variegates and B. tridactylus; B. torquatus is Endangered EN (Restricted to Atlantic Coastal forests of Brazil)

B. pygmaeus is Critically Endangered CR (found on one small island off the Caribbean coast of Panama)

CITES: Choloepus hoffmanni is listed on Appendix III in Costa Rica. Choloepus didactylus is not listed.

Threats to survival

Habitat loss due to ranching, agriculture, urban expansion and logging

Total dependency of forests

habitat fragmentation makes breeding difficult.

Hunting/illegal trade in wildlife

Traffickers buy young sloths from children ($5-$30) who take them from deforested areas. Mothers are frequently killed and sold for bush meat